Cook's note: Stick a wooden pick into the middle of the gratin to test for doneness. The gratin is ready when the pick slides easily through the potatoes.
Preheat oven to 375°. Combine first 4 ingredients in a small bowl.
Melt butter in a Dutch oven over medium-high heat; add onion, and cook, stirring often, 15 minutes or until onions are caramel colored; add garlic, and cook 1 minute.
Stir in flour, and cook, stirring constantly, 1 minute. Gradually whisk in milk and next 3 ingredients. Cook, whisking often, 8 to 9 minutes or until mixture thickens. Remove from heat; whisk in cheese until melted and smooth.
Layer half of potatoes in a lightly greased 13- x 9-inch baking dish; pour 2 cups sauce over potatoes in dish. Repeat layers once.
Bake, uncovered, at 375° for 1 hour and 10 minutes or until golden brown and potatoes are fork tender, topping with breadcrumb mixture during last 15 minutes of baking. Remove from oven, and let stand 10 minutes.
Lightened Potato-Caramelized Onion Gratin: Substitute 1% low-fat milk for whole milk, decrease butter to 1 1/2 Tbsp., and increase flour to 1/4 cup. Proceed with recipe as directed.